I thought that this story on CNN was interesting:
“...scientists have discovered that people can have a love that lasts a lifetime.
Using brain scans, researchers at Stony Brook University in New York have discovered a small number of couples respond with as much passion after 20 years together as most people only do during the early throes of romance, Britain’s Sunday Times newspaper reported.
The researchers scanned the brains of couples together for 20 years and compared them with results from new lovers, the Sunday Times said.
About 10 percent of the mature couples had the same chemical reactions when shown photographs of their loved ones as those just starting out.”